1. Field of the Invention
The present invention relates to a process for removing radioactive ruthenium. More specifically, the present invention relates to a process for separating and removing safely and easily ruthenium, which is a most troublesome nuclide, in nuclear fuel reprocessing.
2. Description of the Related Art
Ruthenium is contained in a large quantity in a fission product. The fission yield of ruthenium varies according to combustion conditions and time after the fission but it is as much as 10% in the case of uranium-235 or even up to 40% in the case of plutonium-239. Since ruthenium-103 (half life of 39.8 days) and ruthenium-106 (half life of 368 days) have a relatively long half life, such radioactive ruthenium is a main radioactive source among radioactive nuclides of the fission product at a certain time after irradiation.
Ruthenium has very peculiar chemical properties and can take any of all of the valencies of 0 to 8. Ruthenium forms various complexes and these complexes have complicated behaviors. Ruthenium tetroxide (RuO.sub.4) in the highest oxidation state is volatile and is volatilized even at room temperature (e.g., to give an ozone smell). Accordingly, ruthenium is a most troublesome nuclide in the reprocessing of a aspent nuclear fuel.
The process most commonly adopted for nuclear fuel reprocessing at the present time is the Purex process. The main step in this process is the extraction separation step where uranium and plutonium are extracted from a solution of spent nuclear fuel in nitric acid with tributyl phosphate (TBP)/n-dodecane and most radioactive fission products are left in the aqueous phase. Accordingly, the disposal of the aqueous phase containing a large quantity of the radioactive nuclide, that is, the highly radioactive liquid waste (HALW), is an important problem. Problems caused by radioactive ruthenium in this reprocessing process are summarized below.
(1) Various complexes formed by ruthenium, especially certain nitrosyl complexes, show complicated distribution behaviors between the aqueous phase and the extract phase in the extraction with TBP/dodecane, and therefore, it is difficult to completely separate ruthenium from uranium and plutonium.
(2) In order to maintain safety, it is intended to concentrate HALW and reduce its volume and to recover nitric acid and use it again. In this case, ruthenium is oxidized by nitric acid, formed ruthenium tetroxide is volatilized and reduced on the reaction vessel wall, and ruthenium dioxide (RuO.sub.2) is precipitated, whereby the radioactive level of the apparatus is increased and corrosion of the vessel wall is drastically promoted, and, in extreme cases, pipes of the apparatus are plugged. Of course, the distilled nitric acid is contaminated with vaporized ruthenium.
(3) HALW is evaporated to dryness and finally vitrified or rendered ceramic by ignition to obtain a stable solid waste. However, volatilization of ruthenium at high temperature cannot be avoided and this volatilization is a main cause of radioactive contamination.
Various measures and studies have been made in order to cope with these problems, but no definitive method for solving these problems has been developed.